I. Field of the Invention
This invention relates to the cooling and guiding of casting belts in apparatus used for continuously casting metal strip articles, particularly twin-belt casters used for casting aluminum alloys and similar metals. The invention also relates to belt casting apparatus incorporating such cooling and guiding equipment.
II. Background Art
The production of metal strip articles, particularly those made of aluminum and aluminum alloys, by twin-belt casting is well known in the art. Casting of this kind involves the use of a pair of endless belts, usually made of flexible but stiffly resilient steel, copper, or the like, which are rotatably driven over appropriate rollers and other path-defining means and supports. The belts define a casting mold formed between moving casting surfaces of confronting generally planar sections of the belts. Molten metal is continuously introduced into the inlet end of the mold via an injector or other feed device, and the metal is cooled as it passes through the mold, to emerge as a continuous metal strip article of desired thickness. A cooling apparatus is generally provided for each belt to provide the necessary cooling effect to cause metal solidification in the mold. Such cooling apparatus may operate by applying a liquid cooling liquid (e.g. water or water with appropriate additives) to the reverse surface of each belt, i.e. the surface opposite to the casting surface in the region of the casting mold, and then withdrawing, and usually recycling, the cooling liquid after it has provided the desired cooling effect. It is also usual in apparatus of this kind to apply a liquid belt dressing, e.g. oil or the like, to the casting surface of each belt before it enters the casting mold. This helps to control the rate of heat transfer from the molten metal to the belts and prevents the molten metal from bonding to the belts.
Twin belt casting apparatus of this kind is disclosed, for example, in U.S. Pat. No. 4,008,750 which issued on Feb. 22, 1977 to Sivilotti et al, U.S. Pat. No. 4,061,178 which issued on Dec. 6, 1977 to Sivilotti et al, U.S. Pat. No. 4,061,177 which issued on De. 6, 1977 to Sivilotti and U.S. Pat. No. 4,193,440 which issued on Mar. 18, 1980 to Thorburn et al. The teaching of these patents is specifically incorporated herein by reference. The ""440 patent discloses an arrangement of belt cooling and guiding means that include generally planar supports for the belts made up of an array of spring-loaded cooling nozzles having hexagonal faces provided with central orifices, from which a cooling liquid is caused to flow under pressure into contact with the reverse surfaces of the belts as they pass through the casting mold. The hexagonal shape of the nozzles means that they may be arranged closely adjacent to each other to form a virtually continuous surface to provide both good support and even cooling effects. However, the nozzles are not quite contiguous so that small gaps remain through which the spent cooling liquid can be drawn under suction from below. The arrangement not only provides cooling, but also helps to hold the belts to the underlying supports by virtue of the vacuum created beneath the belts by the suction means used to withdraw the cooling liquid.
While the above apparatus has proved to be very effective, difficulties have emerged, particularly when apparatus of this kind is used to produce thinner strip articles than those produced conventionally (e.g. strip articles having a thickness in the range of 4 to 10 mm, compared to 10 to 30 mm for conventional castings), and/or those made from alloys having longer freezing ranges (e.g. those having a freezing range of 40 to 50xc2x0 C., compared to up to 20xc2x0 C. for alloys of shorter freezing range). Alloys of long freezing range must be cooled much more quickly and uniformly than alloys of short freezing range to achieve good surface and internal quality plus solidification within the mold. Strip articles of this reduced thickness, and articles made of alloys having longer freezing ranges, are of particular interest to the automotive industry. However, the casting of these alloys and thicknesses requires more controlled casting conditions than can be provided by previous casting cooling systems.
Accordingly, there is a need for improved belt cooling and guiding apparatus and methods so that these problems may be avoided during the use of twin-belt casting apparatus.
An object of the present invention is to improve conventional twin belt casting apparatus so that internal and surface irregularities of the cast strip article and belt deformation may be avoided, particularly when casting thin strip articles or alloys having long freezing ranges.
Another object of the invention is to make the cooling of belts of twin-belt casters more uniform transversely of the belts.
Another object of the invention is to improve the cooling rates (heat flux) that can be achieved in twin-belt casters without causing internal and surface irregularities of the resulting cast strip article, and while avoiding belt deformation.
Another object of the invention is to provide improved belt cooling and guiding means that can be used with twin belt casting apparatus.
The present invention, at least in its main aspect, is based on the finding that, when using twin-belt casting to create thin metal strip products or products of alloys having long freezing ranges, particularly when a liquid belt dressing is applied to the casting surfaces, a very high degree of uniformity of cooling is required transversely of the belts in the region immediately adjacent to the casting mold inlet where the molten metal is first brought into contact with the moving casting surfaces. This degree of uniformity is greater than the degree conventionally obtained with apparatus of the kind described above, and is a consequence of the fact that, in the region where the molten metal is first introduced into the mold, all or a portion of the liquid belt dressing will volatilize and form an insulative gas layer that has a major influence on the heat transfer from the metal to the belt. The uniformity of the volatilization and the insulative gas layer depends on the uniformity of the belt temperature and thus on the uniformity of the belt cooling.
In the present invention, to achieve the desired high degree of transverse temperature uniformity, and desirably a high rate of cooling, cooling liquid is delivered to the reverse side of the belts in this region by means of cooling nozzles having transversely arranged continuous cooling slots, rather than by means of a number of small individual nozzles having one or more discrete delivery openings, or even quasi-linear nozzles having a large number of small openings aligned transversely of the belts.
Thus, according to one aspect of the present invention, there is provided a belt cooling and guiding apparatus for a casting belt of a twin belt caster provided with a pair of rotatably supported endless casting belts, a casting mold formed between moving casting surfaces of confronting generally planar sections of the belts, the sections having reverse surfaces opposite the casting surfaces, the casting mold having a molten metal entrance at one end and a solidified sheet article outlet at an opposite end, and a casting injector for introduction of molten metal into the casting mold at the entrance of the casting mold. The cooling and guiding apparatus comprises at least one elongated nozzle having a support surface facing a reverse surface of the casting belt, a continuous slot in the support surface arranged transversely substantially completely across the casting belt for delivery of cooling liquid to the reverse surface of the belt in the form of a continuous film having a substantially uniform thickness and velocity of flow when considered in the transverse direction of the belt, a drainage opening for removal of cooling liquid at a position spaced from the continuous slot, and a vacuum system associated with the drainage opening for applying suction to the drainage opening. The elongated slot is uninterrupted along its entire length so that there are no barriers to the flow of cooling liquid from the slot.
The apparatus may be produced in the form of an insert for incorporation into existing equipment beneath the casting belts, or may be built into a belt caster as an integral part thereof.
According to another aspect of the invention, there is provided a nozzle for a belt cooling and guiding apparatus, comprising a support surface for supporting a reverse surface of a casting belt, the support surface having a length corresponding to a width of said belt, an elongated continuous slot in said support surface having a length substantially the same as the length of the support surface for delivery of cooling liquid in the form of a continuous film having uniform thickness and velocity of flow along the slot, and a drainage opening for removal of cooling liquid spaced from said continuous slot.
The invention also relates to a twin belt caster of the kind described above incorporating such cooling and guiding apparatus for at least one and preferably both casting belts, positioned at and acting upon the reverse surfaces of the belts.
By the term xe2x80x9ccontinuous slotxe2x80x9d as used herein we mean an elongated orifice in the support surface of the nozzle having no interruptions from one transverse end of the nozzle (relative to the casting belt) to the other. The slot at its inner (cooling liquid entry) side generally opens into a chamber positioned within the nozzle forming a manifold supplied with liquid cooling liquid through inlet passages, the chamber being as wide as the slot is long and having sufficient volume that cooling liquid may be introduced into the chamber through the inlet tubes under pressure and delivered to the open-sided slot with equalized pressure and flow at all points along the length of the slot.
The width (in the direction of advancement of the belt) of the slot of each slotted nozzle is preferably made as small as possible without encountering problems of blockage by particles inevitably present in the cooling liquid. Preferably, the width is in the region of 0.125 to 0.15 mm (0.005 to 0.006 inch). The cooling liquid is preferably filtered thoroughly before being delivered to the nozzle to remove particles that could become trapped in the slot, i.e. particles having a dimension larger than about 0.125 mm.
Preferably the nozzle, or the first such nozzle if more than one is used, is positioned immediately adjacent to the entrance of the casting mold.
By the term xe2x80x9cimmediately adjacent to the entrance of the casting mold,xe2x80x9d we mean that the cooling nozzle(s) provided with the transverse slots are the first cooling means for the belts as the belts advance through the entrance of the casting mold, and that the cooling nozzles extend at the reverse surface of the belt from a position just before and to a distance past the point where molten metal first contacts the belt, such that sufficient heat withdrawal from the molten metal can commence to ensure normal operation of the casting process.
Preferably, there are at least two nozzles provided with such slots for each belt, and more preferably 2 to 4 such nozzles, positioned one following another and extending along the casting mold from the entrance towards the outlet by at least a distance effective to cover the region in which solidification of the molten metal is highly susceptible to transverse variations of the cooling effect (with the first such nozzle preferably positioned immediately adjacent to the entrance of the casting mold). This distance varies from belt caster to belt caster, and for a single belt caster according to the composition of the metal, the cast thickness, the casting speed, the nature of the belt and belt dressing etc., but is often at least 6.6 cm (2.6 inches), incorporating at least two slotted nozzles. If desired, the entire cooling and guiding of each belt may be provided by slotted nozzles arranged one after another along the length of the casting mold, but this is not usually preferable. Once the metal has progressed through the region of extreme sensitivity to cooling variation, the task of further cooling may be taken up by conventional cooling and guiding means (e.g. of the kind disclosed in U.S. Pat. No. 4,193,440 mentioned above), which are generally easier to mount resiliently so as to accommodate cavity convergence for providing continuous support and cooling for the metal as it shrinks during cooling. The first row of such conventional cooling and guiding means should preferably be configured to provide a smooth transition in cooling and support from the slotted nozzles to the conventional nozzles.
Each slotted nozzle of the present invention is preferably bounded on its upstream and downstream edge by a drainage opening (preferably a transverse groove in the support surface for the belt) to receive spent cooling liquid and to remove the liquid from the vicinity of the belt under suction. Each drainage opening or opening is wider (in the direction of advancement of the belt) than the slot of the nozzle next upstream (usually at least 10 times wider) so that rapid and complete withdrawal of spent cooling liquid from the reverse surface of the belt may be achieved. Of course, the width of each drainage opening should not be so great that heat transfer is disrupted due to reduced cooling liquid velocity or sagging of the belt spanning the opening due to lack of adequate support. In general, the drainage openings should have a width of preferably 1.5 to 3 mm.
The slotted nozzles of the present invention not only provide cooling for the casting belts, but also act, to a major extent, as guides for the belts. That is to say, the nozzles provide physical support for the belts, and also act by means of vacuum or suction to hold the belts against perturbations of their positions caused by mechanical or thermal forces. The belts are thus drawn to the nozzle support surfaces to achieve an equilibrium xe2x80x9cstand-offxe2x80x9d (separation) that allows the type of cooling liquid flow described above. This holding action is due partly to the suction applied by the apparatus to remove the cooling liquid from the apparatus, but may also be due in part to a Bernoulli effect created by the cooling liquid flowing over the faces of the slotted nozzles. The nozzles may be designed to optimize this effect, e.g. by suitably profiling the support surfaces of the nozzles in the region of the slot, or at the extreme edges of the support surfaces in the upstream and downstream directions.
The apparatus of the invention is particularly suited for use in belt casters in which a liquid belt dressing (e.g. a volatilizable oil) is applied to the casting surfaces of the belt prior to contact with the molten metal. However, the invention may be operated without the use of liquid belt dressing of this kind.
The present invention can avoid the formation of internal and/or surface defects in the cast article caused by lack of uniform cooling even when casting alloys in thin sections or alloys having a long freezing range.